A gamma ray tool can distinguish between shale and non-shale formations because of the differences in radioactivity between shale and sandstone/carbonate rock. The conventional gamma ray tool measures the number of gamma rays. The spectral gamma ray tool can measure the weight concentrations of potassium, uranium, and thorium in a formation. The azimuthal total gamma ray imaging tool can provide total gamma ray images while the tool is rotating and can associate the total gamma ray measurements with a tool face.
The azimuthal gamma ray imaging tool measures the level of total gamma ray activity of the formation as a function of tool face. This logging-while-drilling (“LWD”) tool can provide high-resolution gamma ray images that identify and define the formation dips, faults and fractures along the horizontal section, even in light of local structural undulations, variations in target zone thickness and lateral property changes along the horizontal section. While the azimuthal gamma ray imaging tool is rotating, an image is formed by stacking the azimuthal gamma measurements along the measured depth.
To make an azimuthally sensitive measurement, however, gamma rays are shielded from the tool. This conflicts with the objective of collecting as many gamma rays as possible to make a precise spectral gamma ray measurement. The precision of spectral gamma ray measurement is then typically poorer than that of a total gamma ray log because only a small portion of gamma rays can be used to infer formation potassium, uranium, and thorium. In addition, generally, to make spectral gamma ray measurement, larger detectors, bigger in size, are used in comparison to gamma ray imaging. Therefore, the space that shielding occupies in gamma ray imaging tool could be used to mount a large detector to improve detection efficiency.
A need exists, therefore, for systems and methods that can measure gamma rays without the tool or detector to being collimated and/or shielded in order to make azimuthally sensitive elemental measurements.